Quick restart of fuel cell power plant as alternative to idling

ABSTRACT

A fuel cell power plant keeps track, such as with a fuel-off timer ( 41 ), of the extent to which shutdown of the fuel cell power plant has occurred, in case the fuel cell power plant is quickly commanded to resume full operation. In one embodiment, if the fuel-off timer has not timed out at the time that the fuel cell power plant is ordered to resume full operation, a fuel-on timer is set ( 51 ) equal to the value of the fuel-off timer when the fuel cell power plant is ordered to resume full operation. Then, the fuel cell power plant is refueled ( 22 ), in a duration of time related to the setting of the fuel-off timer, rather than doing a full fuel purge.

TECHNICAL FIELD

A fuel cell power plant, such as one providing electricity to power anelectric vehicle, that are frequently allowed to idle when little or nopower is being required in order to avoid the two or three minutestypically required to restart the fuel cell power plant is provided withquick restart when having been shut down only a short time before, toreduce the need to idle.

BACKGROUND ART

Proton exchange membrane fuel cells are considered to be quite suitablefor applications in which the fuel cell power plant will provideelectricity to the electric motor of a vehicle. In some applications,such as city buses and package delivery trucks, the vehicle makesfrequent stops, mostly for short periods of time. Whenever the electricmotor is demanding little or no power from the fuel cell, the currentdensity is very low and the voltage of the cells rise close to opencircuit potentials. Although charging of batteries can assist inavoiding the increased potentials, that cannot happen when the state ofcharge is high.

A practice of shutting down the fuel cell, rather than letting it idle,has seldom been used because once the fuel cell is shut down, it takes afew minutes to get it into full operation again. Thus, the continuedpreference among operators is for idling, even though oxidation of thecatalyst, particularly at the cathode, will occur.

SUMMARY

To avoid idling, what is needed is a quick restart function tofacilitate restart whenever the vehicle provides a start command shortlyafter the fuel cell power plant has been commanded to shut down.Providing the control system of the fuel cell power plant with theability for a rapid restart primarily requires reducing the timenecessary a) to get the thermal management system operating and b) toaccomplish all or part of the fuel supply startup procedure relevant toa normal startup.

In one embodiment, the time since initiating turn off of the fuel systemduring a shutdown is monitored, so that if restart is commanded soonthereafter, the extent of fuel purge can be made commensurate(proportionately brief), rather than having it be the same as it wouldduring a normal startup after a much longer shut-down period, such as atthe start of an operational day. In one embodiment, the thermalmanagement (or temperature management) system is not turned off during ashutdown until the fuel system has had adequate time to be fully turnedoff. This makes it less likely that the fuel management system will haveto be totally filled unless an extensive shutdown will occur.

Other variations will become more apparent in the light of the followingdetailed description of exemplary embodiments, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified functional diagram illustrating an embodiment ofquick restart of a fuel cell power plant.

FIG. 2 is a simplified version of FIG. 1, illustrating the quick startfeature more clearly.

MODE(S) OF IMPLEMENTATION

Referring to FIG. 1, a method 11, for assisting in the quick restart ofa fuel cell power plant, has two stable modes: (1) a fuel cell standbymode 12 when the fuel cell is turned off and the controller of the fuelcell is waiting for a vehicle request for the fuel cell power plant torun and (2) a ready-for-load mode 13. Because there are significantdifferences between the major control loops of various fuel cell powerplants, the illustration in the figure has been simplified by omittingevents that indicate completion of any particular step or requiredcondition. For instance, adequate cathode gas recycle may be achieved inone power plant by proper valve adjustments and blower operation for agiven predetermined period of time. Alternatively, a hydrogen sensor inthe cathode gas stream may be used as an indicator that the cathoderecycle to consume oxygen gas has been completed. The manner of causingstep advancements is not germane to the quick restart herein. It isassumed that all such particular details will be easily accommodatedonce the exemplary embodiment of quick restart is apparent.

So long as a test 14 is negative, the fuel cell power plant will remainin the standby mode 13. Once the vehicle provides a run request, avehicle run request flag is set equal to “1” and test 14 is affirmative.Other manifestations of the need for vehicle power may be used.

To get the fuel cell power plant into operation, a first step may be toturn on the temperature management (or thermal management) system in astep 17. The temperature management system start decision variesdepending on the characteristics of the fuel cell power plant, notgermane to the disclosure herein. The temperature management system maynot be turned on until later in the start up process, or to supportreaching operating temperature, or for some other reason.

Next, a step 19 will initiate the setting in a time out timer,identified as a fuel-on tinier. The timer is set to a value which willvary as a function of the fuel cell power plant's operational state whenthe start request is received. The operational state varies from beingstarted up from having been fully shut down, in contrast with beingstarted up using a quick restart process, as described more fullyhereinafter. The time set in the timer will be such as to provide thenecessary fuel purge.

A step 22 initiates a process of filling the anode and fuel recycleplumbing with fuel. An associated test 23 determines if the fuel-ontimer has timed out or not; so long as it has not, the fueling inresponse to step 22 will continue. Once the fuel-on timer has timed out,an affirmative result of test 23 reaches a step 25 which causes theblower to start to fill the cathode with air. Then, a step 28 can closethe main load switch so as to advance the status of the fuel cell powerplant to a mode 29, ready for a load, where the fuel cell power plant isprepared to satisfy any load from zero load to its maximum load. In thismode, normal operation occurs.

For shutting down the fuel cell power plant, the vehicle run requestflag is set to “0”. This is monitored in a test 32 which retains thefuel cell power plant in the ready for load mode 29 unless the vehiclerun request flag is set to “0”. An affirmative result of test 32 reachesa step 34 which opens the main load switch and connects the stack tovoltage limiting devices. The voltage limiting devices will assist inpreventing uncontrolled open circuit cell potentials. A step 36 willcause the cathode exhaust to be recycled to the entrance of the cathodeflow fields, so as to reduce oxygen content therein. Oxygen is consumedas the oxygen combines with hydrogen which crosses over from the anodethrough the membrane and by the cell current drawn through the stack'svoltage limiting devices. Once a predetermined interval of cathoderecycle has occurred, the air blower is turned off by a step 37.

Following air blower shutdown, a fuel-off, time-out timer is set to azero count in a step 41. Contemporaneously with step 41 is the turn offof the fuel cell power plant's fuel cell fuel system, in a step 42. Whena quick restart procedure is to be used, the characteristics of fuelcell power plant will determine the process used for turning the fuelsystem off, primarily the length of time necessary to shut off the fuelsystem. During the time the fuel system is shutting off, a test 44monitors whether a quick restart is being requested (or not) by testingthe vehicle run request flag for a “1”. If the run request flag is notequal to “1”, a negative result of test 44 initiates a test 46 todetermine if the fuel-off timer has timed out. If test 46 is negative,the system returns to test 44. Until time out of the fuel-off timer isreached, if a request to start is indicated by the vehicle run requestflag equaling a “1”, an affirmative result of test 44 will initiate astep 49 which stops the fuel-off timer. The run time of the fuel-offtimer indicates the extent to which the fuel system has completed itsturn-off routine. The run time of the fuel-off timer is then used toinitiate the fuel-on, time-out timer. The setting for the fuel-on,time-out timer will be a function of the setting in the fuel-off timer,in a step 51. The function can be related to the time established in thefuel-off timer, but may include other parameters such as ambienttemperature or pressure, cell stack assembly temperature, elevation ofthe site, and so forth.

Following step 51, the routine reverts to the step 22 which commandsthat the anode and recycle plumbing be filled with fuel. The step 22command is modified so that, instead of utilizing the full duration oftime which is required upon an initial startup, a lesser amount of timeis used for a quick start-up when the fuel-off timer is set to a lessertime period. The step 23 is testing for a fuel-on time out which is setas a function of the fuel off timer in the step 51. As a result, a fullfuel purge may not be required, saving additional startup time.

In the case where the routine advances from the step 51 to the step 22,the temperature management system has not been turned off, thus savingadditional start time.

If a vehicle run request is not sensed by the flag equaling “1” in thetest 44 before the test 46 determines that the fuel-off timer has timedout, an affirmative result of test 46 causes a step 54 to turn off thethermal management system. This causes reversion to the fuel cellstandby mode 13 where the routine will wait until a vehicle request forthe fuel cell to run is indicated by the flag of test 14 being set to a“1”.

If the moment the step 54 orders turning off of the thermal managementsystem, a vehicle run request is manifested by setting the flag of test14 equal to a “1”, response will be immediate so that the step 17 willbe reached prior to any loss of time with respect to the thermalmanagement system. Stated alternatively, the thermal management systemcan be restored to normal operation directly from any stage, andrequires less time depending upon how soon the order to turn it off iscountermanded by an order to turn it on. Therefore, this saves time aswell.

The invention claimed is:
 1. A method, comprising: a) holding a standbymode until there is selectively provided an indication of the fact thata vehicle having an electric propulsion system powered by a fuel cellpower plant currently requires fuel cell system electric power; b) afterstandby mode, bringing the fuel cell power plant into a ready-for-loadmode in which said fuel cell power plant is capable of providing anyload demanded of said fuel cell power plant between no load and amaximum load, including: i) turning on a fuel cell thermal managementsystem of the fuel cell power plant, ii) initializing a fuel-on timer toa value determined as a function of an operational state of the fuelcell power plant, and iii) filling anodes and anode recycle plumbing ofthe fuel cell power plant with fuel until the fuel-on timer times out;c) after achieving ready-for-load mode, remaining in the ready-for-loadmode until there is selectively provided an affirmative result, from afirst vehicle run test, a symbol of the fact that the vehicle currentlyrequires no electric power; d) after the affirmative result isselectively provided, simultaneously initiating setting a fuel-off timerto zero and initiating a turn-off of a fuel cell fuel system of the fuelcell power plant; and e) upon a positive result for a second vehicle runtest during a duration that precedes time-out of the fuel-off timer,stopping the fuel-off timer and setting the fuel-on timer equal to thefuel-off timer; and f) returning to the filling of the anodes and anoderecycle plumbing of the fuel cell power plant with fuel until thefuel-on timer times out.
 2. A method according to claim 1, furthercomprising: between time-out of the fuel-on timer and the ready-for-loadmode, starting an air blower and filling cathodes of the fuel cell powerplant with air, and closing a main load switch to connect the fuel cellpower plant to the vehicle.
 3. A method according to claim 2, furthercomprising: in response to said symbol, opening the main load switch todisconnect the fuel cell power plant from the vehicle, connecting atleast one voltage limiting device to a fuel cell stack power output ofthe fuel cell power plant, recycling of cathode exhaust to cathode flowfield inlets, and turning off the air blower.
 4. A method according toclaim 1, further comprising: after the fuel-on timer times out, iv)starting an air blower and filling cathodes of the fuel cell power plantwith air, and v) closing a main load switch to connect the fuel cellpower plant to the vehicle; and wherein initiating the turn-off of thefuel cell fuel system includes opening the main load switch todisconnect the fuel cell power plant from the vehicle and connecting atleast one voltage limiting device to a fuel cell stack power output ofthe fuel cell power plant, and recycling cathode exhaust to cathode flowfield inlets and turning off the air blower.
 5. A method according toclaim 1, further comprising: upon time-out of the fuel-off timer,turning off the fuel cell thermal management system.
 6. A method ofoperating a fuel cell, comprising: filling anodes of the fuel cell withfuel using a fuel system of the fuel cell until the fuel cell is capableof providing a maximum output of the fuel cell; simultaneouslyinitiating a fuel-off timer and initiating a turn-off of the fuelsystem; and simultaneously setting a fuel-on timer equal to the fuel-offtimer and initiating filling the anodes of the fuel cell with fuel for aduration of the fuel-on timer.